How to use try catch block with transaction?

begin try

    begin transaction

    -- alter table z drop column aaa;
    alter table z add zzz int;

    commit tran;

end try
begin catch

    print 'hello';
    SELECT
        ERROR_NUMBER() as ErrorNumber,
        ERROR_MESSAGE() as ErrorMessage;

    IF (XACT_STATE()) = -1
    BEGIN
        PRINT
            N'The transaction is in an uncommittable state. ' +
            'Rolling back transaction.'
        ROLLBACK TRANSACTION;
    END;

end catch

print 'reached';